Power converter using multiple controllers

ABSTRACT

A power converter controller includes a primary controller and a secondary controller. The primary controller is coupled to receive one or more request signals from the secondary controller and transition a power switch from an OFF state to an ON state in response to the received request signals. The secondary controller is coupled to transmit the request signals to the primary controller and control the amount of time between the transmission of each of the request signals. The secondary controller includes a timing circuit that sets a minimum amount of time between the transmission of the request signals. The secondary controller also includes a secondary switch control circuit coupled to trigger the timing circuit in response to transmitting a request signal.

This application is a continuation of U.S. patent application Ser. No.13/791,459, filed Mar. 8, 2013, now pending. U.S. patent applicationSer. No. 13/791,459 is hereby incorporated by reference.

BACKGROUND INFORMATION

1. Field of the Disclosure

This disclosure relates to power supplies and, more particularly, tocontrol circuits for power supplies.

2. Background

Switch mode power supplies are widely used in household or industrialappliances for converting a low frequency (e.g., 50 Hz or 60 Hz) highvoltage alternating current (ac) input voltage to a required level ofdirect current (dc) output voltage. For example, switch mode powersupplies may be included in electronic devices such as battery chargersfor mobile electronic devices. Various types of switch mode powersupplies are popular because of their well-regulated output, highefficiency, and small size along with their safety and protectionfeatures. Popular topologies of switch mode power supplies includeflyback, forward, boost, buck, half bridge, and full bridge, among manyothers including resonant types.

Switch mode power supplies may include an energy transfer element, apower switch, and control circuits that operate to regulate the value ofthe power converter output voltage. The energy transfer element (e.g., acoupled inductor) may include a primary winding and a secondary windingthat are galvanically isolated from one another. The primary winding maybe coupled to circuits on the input side of the power converter, such asthe power switch. The secondary winding may be coupled to circuits onthe output side of the power converter that deliver the regulated outputvoltage to the electrical load.

The power switch (e.g., a high voltage power switch) may be coupled tothe primary winding of the energy transfer element to control currentthrough the primary winding. The control circuits of the power convertermay sense the output voltage and control the state of the power switchto control the transfer of energy from the primary winding to thesecondary winding in response to the sensed output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present disclosureare described with reference to the following figures, wherein likereference numerals may refer to like parts throughout the variousfigures.

FIG. 1 is a schematic diagram of an example power converter including aprimary controller, a secondary controller, and a power switch.

FIG. 2 is a functional block diagram of an example integrated circuitpackage that includes a primary controller, a secondary controller, anda power switch.

FIG. 3 is a flow diagram describing operation of an example secondarycontroller of a power converter.

FIG. 4 is a flow diagram describing operation of an example primarycontroller of a power converter.

FIG. 5 is a flow diagram describing operation of an example primarycontroller and an example secondary controller of a power converter.

FIG. 6 illustrates various waveforms generated during operation of anexample primary controller and an example secondary controller.

FIG. 7A is a functional block diagram of an example integrated circuitpackage that includes a magnetically coupled communication link.

FIG. 7B is a functional block diagram of an example integrated circuitpackage that includes an optically coupled communication link.

FIG. 8 is a schematic diagram of an example non-isolated power converterthat includes an example primary controller and an example secondarycontroller.

FIG. 9 illustrates various alternate waveforms generated duringoperation of an example primary controller and an example alternativesecondary controller.

Corresponding reference characters may indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. Common butwell-understood elements that are useful or necessary in commerciallyfeasible embodiments are often not depicted in order to facilitateunderstanding of the various embodiments.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures, or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. Particular features, structures, or characteristics may beincluded in an integrated circuit, an electronic circuit, acombinational logic circuit, or other suitable components that providethe described functionality.

A power converter according to the present disclosure includes a primarycontroller and a secondary controller that are galvanically isolatedfrom one another, e.g., by a communication link. The primary controllermay be coupled to control a state of a power switch to control thetransfer of energy from an input of the power converter to an output ofthe power converter. The secondary controller may be coupled to circuitcomponents at the output of the power converter in order to sense anoutput quantity of the power converter. Although the primary controllerand the secondary controller are galvanically isolated from one another,the secondary controller may transmit signals to the primary controllerto control how the primary controller switches the power switch. Forexample, the secondary controller may transmit signals to the primarycontroller in response to a sensed output quantity of the powerconverter.

In some examples, the primary and secondary controllers of the presentdisclosure may be included in an isolated power converter (e.g., aflyback converter) in which the input terminals of the isolated powerconverter are galvanically isolated from the output terminals of theisolated power converter by an energy transfer element (e.g., a coupledinductor). In these examples, the primary controller may be coupled tocircuits on the primary side of the isolated power converter, such as apower switch. The secondary controller may be coupled to circuits on thesecondary side of the isolated power converter to sense an outputquantity of the isolated power converter.

In some examples, the primary and secondary controllers of the presentdisclosure may be included in a non-isolated power converter (e.g., anon-isolated buck converter) in which the input terminals of thenon-isolated power converter are not galvanically isolated from theoutput terminals. When used in a non-isolated power converter, theprimary and secondary controllers may be galvanically isolated from oneanother (e.g., by a communication link) although the input terminals andoutput terminals of the non-isolated power converter are notgalvanically isolated from one another.

The primary and secondary controllers may operate to regulate an outputquantity (e.g., voltage and/or current) of the power converter that isdelivered to a load. For example, the primary and secondary controllersmay operate to regulate the output voltage of the power converter to adesired output voltage value in response to a sensed output voltage.Although the primary and secondary controllers may regulate the outputvoltage in response to a sensed output voltage, in some examples, theprimary and secondary controllers may regulate the output voltage and/orthe output current of the power converter in response to a sensed outputvoltage and/or a sensed output current.

The secondary controller is coupled to transmit an energy request signal(hereinafter “request signal”) to the primary controller. The primarycontroller is coupled to set the power switch into an ON state (e.g., aclosed switch) when the primary controller receives the request signal.Accordingly, the secondary controller of the present disclosure maycontrol when the power switch is set into the ON state. After theprimary controller sets the power switch into the ON state, the primarycontroller determines when to set the power switch into the OFF state(e.g., an open circuit). Accordingly, the primary controller may controlwhen the power switch is turned OFF. In other words, the primarycontroller may control how long the power switch remains in the ONstate. As described hereinafter, the secondary controller may include atiming circuit that controls how often the power switch may be set intothe ON state by the primary controller. Put another way, the secondarycontroller may control the rate (e.g., the max rate) at which the powerswitch is set into the ON state.

The primary controller may include circuits that set the state of thepower switch (e.g., a primary switch control circuit 250 of FIG. 2). Ingeneral, the primary controller may maintain the power switch in an OFFstate until a request signal is received from the secondary controller.In response to the request signal, the primary controller may set thepower switch into an ON state. After setting the power switch in the ONstate, the primary controller may determine when to set the switch intothe OFF state in response to one or more of a variety of differentconditions, referred to herein as “turn-off conditions.” In someexamples, the primary controller may sense the switch current throughthe power switch and set the power switch into an OFF state when theswitch current reaches a threshold current limit while the power switchis in the ON state. In other examples, the primary controller may becoupled to set the power switch into the ON state for a set period oftime, referred to herein as a “conduction period.” The primarycontroller may set the power switch into the OFF state after theconduction period has expired. The turn-off conditions (e.g., thresholdcurrent limit and/or the conduction period) may be fixed quantities insome examples. In other examples, the primary controller may adjust theturn-off conditions, e.g., in response to loading conditions.

The secondary controller includes circuits that control when requestsignals are transmitted to the primary controller. For example, thesecondary controller may include a timing circuit and a secondary switchcontrol circuit (e.g., the secondary switch control circuit 256 of FIG.2) that control how often request signals are sent to the primarycontroller, which in turn controls how often the power switch is setinto an ON state. The secondary switch control circuit may generate arequest signal in response to a sensed output quantity of the powerconverter (e.g., output voltage) and a state of the timing circuit, asdescribed hereinafter.

The timing circuit may operate in one of a first state and a secondstate. In general, the timing circuit may operate in a first state untilthe timing circuit is triggered by the secondary switch control circuitto operate in the second state. As described herein, when the timingcircuit is in the second state, the secondary switch control circuit maybe prevented from transmitting a request signal. When triggered by thesecondary switch control circuit, the timing circuit may transition fromthe first state to the second state and remain in the second state for aperiod of time, referred to herein as a “holding period.” After thetiming circuit has been in the second state for a holding period, thetiming circuit may transition back to the first state. The timingcircuit may stay in the first state until triggered by the secondaryswitch control circuit, as described above.

The secondary switch control circuit is coupled to sense the outputvoltage of the power converter and determine whether the output voltageis less than a desired output voltage. The secondary switch controlcircuit may transmit a request signal to the primary controller andtrigger the timing circuit when the sensed output voltage is less than adesired output voltage and the timing circuit is in the first state. Forexample, the secondary switch control circuit may transmit a requestsignal to the primary controller and also generate a trigger signal thattriggers the timing circuit. Since the primary controller sets the powerswitch into the ON state in response to the request signal, the timingcircuit may transition to the second state at approximately the sametime as the power switch is set into the ON state.

As described above, the secondary switch control circuit may transmit arequest signal when the secondary switch control circuit determines thatthe output voltage is less than the desired output voltage anddetermines that the timing circuit is in the first state. In othercircumstances, such as when the timing circuit is in the second state orthe output voltage is greater than the desired output voltage, thesecondary switch control circuit may withhold transmission of therequest signal. In other words, the secondary switch control circuit maydecide not to transmit the request signal when the timing circuit is inthe second state and/or the output voltage is greater than the desiredoutput voltage.

In one example, when the output voltage of the power converter isgreater than the desired output voltage and the timing circuit is in thefirst state, the secondary switch control circuit may withholdtransmission of the request signal until the output voltage drops to avalue that is less than the desired output voltage. In this example, thesecondary switch control circuit may transmit the request signal andtrigger the timing circuit when the output voltage drops to less thanthe desired output voltage. In another example, when the output voltageis less than the desired output voltage and the timing circuit is in thesecond state, the secondary switch control circuit may withholdtransmission of the request signal until the timing circuit transitionsto the first state. In this example, the secondary switch controlcircuit may transmit the request signal and trigger the timing circuitback to the second state in response to the timing circuit entering thefirst state, assuming that the output voltage is still at a level thatis less than the desired output voltage when the timing circuittransitions to the first state.

The secondary switch control circuit may control the rate at whichrequest signals are transmitted to the primary controller. Accordingly,the secondary switch control circuit may control the rate at which thepower switch is set to the ON state since the primary controller may setthe power switch in the ON state in response to each request signal. Thesecondary switch control circuit may control the rate at which requestsignals are sent in response to an amount of loading at the output ofthe power converter. For example, the secondary switch control circuitmay tend to transmit request signals at a greater rate during heavierloading when the output voltage of the power converter may tend to dropbelow the desired output voltage faster than during lighter loading atthe output. In examples where the loading at the output decreases, thesecondary switch control circuit may tend to transmit request signals ata lower rate than when heavier loading is present at the output.

The holding period of the timing circuit may set a maximum rate at whichrequest signals may be transmitted since the secondary switch controlcircuit withholds transmission of a request signal while the timingcircuit is in the second state. Accordingly, the holding period of thetiming circuit may set a maximum rate at which the power switch may beset into the ON state. Put another way, the holding period may beapproximately equal to the minimum time between two consecutive requestsignals, or two consecutive transitions of the power switch into the ONstate. For example, during heavy loading, the secondary switch controlcircuit may transmit request signals that are separated by approximatelyone holding period of the timing circuit.

The holding period may be set to a value that allows a sufficient amountof time for energy to be transferred to the output side of the powerconverter. In examples where the power converter is an isolated powerconverter, the holding period may be set to a value that allows forenergy transfer to the secondary side after the primary controller hasswitched the power switch from the ON state to the OFF state. Since theprimary controller determines how long the power switch remains in theON state, the holding period of the timing circuit and the turn-offconditions of the power switch may be selected such that a sufficientamount of energy is transferred after the power switch is set to the OFFstate.

The primary controller may adjust the turn-off conditions (e.g.,threshold current limit and/or the conduction period) in response toloading conditions at the output of the power converter. For example,the primary controller may adjust the turn-off conditions based on therate at which the primary controller receives request signals. Asdescribed above, the primary controller may tend to receive requestsignals at a greater rate during times when the output is more heavilyloaded. In some examples, the primary controller may determine loadingconditions based on how many request signals the primary controllerreceives over a period of time. In other examples, the primarycontroller may determine loading conditions based on the amount of timebetween two consecutive request signals.

In general, the primary controller may adjust the turn-off conditions sothat the power switch is kept in the ON state for a longer period oftime when loading is heavier at the output. For example, the primarycontroller may increase the threshold current limit and/or theconduction period of the power switch during heavier loading so that thepower switch is held in the ON state for a greater amount of time.Holding the power switch in the ON state for a greater amount of timemay result in a greater amount of energy transfer to the output of thepower converter so that the output voltage of the power converter ismaintained at the desired output voltage during heavier loading.

Example power supplies according to the present disclosure are nowdescribed with reference to FIGS. 1-10. FIG. 1 shows an example isolatedpower converter that includes a primary controller and a secondarycontroller that operate to regulate an output quantity of the isolatedpower converter to a desired output quantity. FIG. 2 illustrates a moredetailed view of example primary and secondary controllers. FIGS. 3-5are flow diagrams that illustrate operation of example primary andsecondary controllers. FIG. 6 illustrates various waveforms generatedduring operation of the primary and secondary controllers. FIGS. 7A-7Bshow example communication links through which the secondary controllermay send a request signal to the primary controller. FIG. 8 shows anon-isolated power converter that includes example primary and secondarycontrollers. FIGS. 9-10 describe operation of an alternative secondarycontroller.

FIG. 1 is a schematic diagram of an example power converter 100according to the present disclosure. Example power converter 100 is anisolated switch mode power converter having a flyback topology. Althoughprimary side control circuit 118 and secondary side control circuit 120of FIG. 1 are included in an isolated power converter, in otherexamples, primary side control circuit 118 and secondary side controlcircuit 120 may be included in non-isolated power supplies (e.g., anon-isolated buck converter as shown in FIG. 8).

Power converter 100 includes input terminals 102-1, 102-2 (collectively“input terminals 102”) and output terminals 104-1, 104-2 (collectively“output terminals 104”). Input terminals 102 are coupled to receive aninput voltage V_(IN) 106, which may be a rectified and filtered acvoltage. For example, input terminals 102 may be coupled to afull-bridge rectifier (not shown) and a filter capacitance (not shown)that are coupled to rectify and filter an ac voltage received from an acvoltage source. In one example, input voltage V_(IN) 106 may be atime-varying dc voltage. As shown, V_(IN) 106 is referenced to inputterminal 102-2, which may be referred to as an “input return 102-2.”

Output terminals 104 provide an output voltage V_(OUT) 108 to anelectrical load (not shown). After startup of power converter 100, powerconverter 100 may regulate the value of output voltage V_(OUT) 108 to adesired output voltage value (e.g., 5 to 12 V dc). Startup may be aperiod of time starting from when power converter 100 is introduced toV_(IN) 106 until the control circuits of power converter 100 beginoperating to regulate the output voltage V_(OUT) 108 of power converter100. Accordingly, output voltage V_(OUT) 108 may be referred to as a“regulated output voltage.” Output terminals 104 are coupled to anoutput capacitor 110 to smooth out regulated output voltage V_(OUT) 108.As shown, output voltage V_(OUT) 108 is referenced to output terminal104-2, which may be referred to as an “output return 104-2.”

Power converter 100 includes an energy transfer element 112. Energytransfer element 112 includes a primary winding 114 and a secondarywinding 116. Energy transfer element 112 is coupled to transfer energyfrom primary winding 114 to secondary winding 116. In one example,energy transfer element 112 may be a coupled inductor. Circuits that areelectrically coupled between input terminals 102 and primary winding 114may be referred to as the “primary side” of power converter 100.Circuits that are electrically coupled between secondary winding 116 andoutput terminals 104 may be referred to as the “secondary side” of powerconverter 100. Energy transfer element 112 provides galvanic isolationbetween circuits on the primary side of power converter 100 and circuitson the secondary side of power converter 100. Accordingly, a dc voltageapplied between the primary side and the secondary side of powerconverter 100 will produce substantially zero current.

Power converter 100 includes a primary side control circuit 118(hereinafter “primary controller 118”), a secondary side control circuit120 (hereinafter “secondary controller 120”), and a power switch 122.Primary controller 118, secondary controller 120, and power switch 122are included in an integrated circuit package 124, which is illustratedas a box in FIG. 1.

In one example, integrated circuit package 124 may include a firstintegrated circuit die and a second integrated circuit die within anencapsulation. An encapsulation may refer to an encasing or molding thatsurrounds or encloses one or more integrated circuit dice and a portionof a lead frame. The first integrated circuit die may include primarycontroller 118 and power switch 122. The second integrated circuit diemay include secondary controller 120. In another example, integratedcircuit package 124 may include three integrated circuit dice within anencapsulation. For example, integrated circuit package 124 may include afirst integrated circuit die that includes power switch 122, a secondintegrated circuit die that includes primary controller 118, and a thirdintegrated circuit die that includes secondary controller 120.

The integrated circuit dice including primary controller 118 andsecondary controller 120 are galvanically isolated from one another.Accordingly, secondary controller 120 is galvanically isolated fromprimary controller 118 and power switch 122. Although primary controller118 and secondary controller 120 are galvanically isolated from oneanother, primary controller 118 and secondary controller 120 maycommunicate with one another. In one example, secondary controller 120may communicate with primary controller 118 via a communication link. Inone example, the communication link may be a magnetically coupledcommunication link. An example magnetically coupled communication linkis described with reference to FIG. 7A). In another example, secondarycontroller 120 may communicate with primary controller 118 through anoptically coupled communication link. An example optically coupledcommunication link is described with reference to FIG. 7B). In otherexamples, secondary controller 120 may communicate with primarycontroller 118 through other types of communication links, such as acapacitive communication link.

Although primary controller 118, secondary controller 120, and powerswitch 122 are illustrated as included in a single integrated circuitpackage, in other examples, one or more of primary controller 118,secondary controller 120, and power switch 122 may be located outside ofthe illustrated integrated circuit package. For example, power switch122 may be included in an integrated circuit package that is separatefrom another integrated circuit package that includes both primarycontroller 118 and secondary controller 120.

Circuits external to integrated circuit package 124 may electricallycouple to package terminals D 126-1, S 126-2, PBP 126-3, FWD 126-4, BP126-5, GND 126-6, and FB 126-7 (collectively “package terminals 126”) ofintegrated circuit package 124. Package terminals 126 of integratedcircuit package 124 may include conductive pins and/or conductive padsfor connection to circuits external to integrated circuit package 124.

Package terminals 126 may connect to terminals (e.g., on integratedcircuit die) of power switch 122, primary controller 118, and secondarycontroller 120 included inside encapsulation of integrated circuitpackage 124. Power switch 122 includes terminals D 128-1 and S 128-2.Primary controller 118 includes terminal PBP 128-3. Secondary controller120 includes terminals FWD 128-4, BP 128-5, GND 128-6, and FB 128-7.Terminals D 128-1, S 128-2, PBP 128-3, FWD 128-4, BP 128-5, GND 128-6,and FB 128-7 may be conductive connections included on the integratedcircuit die that include power switch 122, primary controller 118, andsecondary controller 120. GND terminal 128-6 is coupled to outputterminal 104-2. In one example, GND terminal 128-6 may be the outputreturn for secondary controller 120.

Primary controller 118 is coupled to circuit components of the primaryside of power converter 100, such as power switch 122. Secondarycontroller 120 is coupled to circuit components of the secondary side ofpower converter 100. For example, secondary controller 120 may becoupled to secondary winding 116 and a bypass capacitor 130. Secondarycontroller 120 may also be coupled to output terminals 104 via feedbackcircuits (not shown) that allow secondary controller 120 to sense anoutput quantity of power converter 100 (e.g., output voltage V_(OUT) 108and/or output current IouT 109). For example, power converter 100 ofFIG. 1 may include feedback circuits between output terminals 104 andfeedback terminal FB 126-7 that generate a feedback voltage VFB 132 thatis representative of output voltage V_(OUT) 108. Although powerconverter 100 of FIG. 1 may include feedback circuits that generatefeedback voltage VFB 132, in other examples, power converter 100 mayinclude circuits that generate a feedback current that is representativeof output current IouT 109. Primary controller 118 and secondarycontroller 120 control circuits of power converter 100 (e.g., powerswitch 122) to control energy transfer from input terminals 102 tooutput terminals 104.

Secondary controller 120 receives power from the secondary side of powerconverter 100. For example, secondary controller 120 may receive powerfrom bypass capacitor 130 which is coupled to secondary controller 120at bypass terminal BP 128-5 and ground terminal GND 128-6. Bypasscapacitor 130 may supply power to circuits of secondary controller 120such as timing circuit 258 (FIG. 2) and secondary switch control circuit256 (FIG. 2). Secondary controller 120 may include circuits thatregulate bypass voltage VBP 134 across bypass capacitor 130 (e.g., atapproximately 4 to 5 V). In some examples, secondary controller 120 mayinclude circuits that charge bypass capacitor 130 from forward terminalFWD 128-4, e.g., during startup and operation.

Although primary controller 118 and secondary controller 120 aregalvanically isolated from one another, secondary controller 120 maytransmit an energy request signal U_(REQ) 136 (hereinafter “requestsignal U_(REQ) 136”) to primary controller 118. For example, secondarycontroller 120 may transmit request signal U_(REQ) 136 via acommunication link, e.g., a magnetic, capacitive, or an opticalcommunication link. As described herein, primary controller 118 may setpower switch 122 into an ON state in response to a request signalU_(REQ) 136 received from secondary controller 120.

Power switch 122 may be a high voltage power switch, which may have abreakdown voltage in the range of 700-800 V. In one example, powerswitch 122 may be a power metal-oxide-semiconductor field-effecttransistor (power MOSFET), as illustrated in FIG. 2. Power switch 122 iscoupled to primary winding 114 and input return 102-2. In examples wherepower switch 122 is a power MOSFET, the drain of the power MOSFET may becoupled to drain terminal D 128-1 and the source of the power MOSFET maybe coupled to source terminal S 128-2, as illustrated in FIG. 2.

Primary controller 118 controls current through power switch 122 andprimary winding 114 by controlling the state of power switch 122.Current through power switch 122 may be referred to herein as “switchcurrent.” In general, power switch 122 may be in an “ON” state (e.g., aclosed switch) or an “OFF” state (e.g., an open switch), in response toa switch drive signal U_(DRIVE) 138 generated by primary controller 118.When power switch 122 is in the ON state (e.g., a closed switch), powerswitch 122 may conduct current. When power switch 122 is in the OFFstate (e.g., an open switch), power switch 122 may not conduct currentwhen a voltage is applied across power switch 122.

Primary controller 118 generates switch drive signal U_(DRIVE) 138 tocontrol the state of power switch 122. In an example where power switch122 is a power MOSFET, primary controller 118 may be coupled to the gateof the power MOSFET, as illustrated in FIG. 2. In this example, primarycontroller 118 applies a gate-to-source voltage that is greater than thethreshold voltage of the power MOSFET to set the power MOSFET into theON state. Primary controller 118 applies a gate-to-source voltage thatis less than the threshold voltage of the power MOSFET to set the powerMOSFET into the OFF state.

Primary controller 118 receives operating power from input terminals 102and/or primary bypass capacitor 140. In one example, primary controller118 may also receive operating power from a low voltage winding (notshown in FIG. 1) forming part of energy transfer element 112. Primarybypass capacitor 140 may store energy received from input terminals 102when input voltage V_(IN) 106 is provided at input terminals 102. Energystored on primary bypass capacitor 140 may be used as operating power byprimary controller 118, e.g., to generate switch drive signal U_(DRIVE)138.

When power switch 122 is in the ON state, current through primarywinding 114 increases, storing energy in energy transfer element 112.Additionally, a primary winding voltage V_(P) 142 with a first polaritydevelops across primary winding 114 while power switch 122 is in the ONstate. A secondary winding voltage V_(S) 144 of opposite polarity withrespect to primary winding voltage V_(P) 142 develops across secondarywinding 116 while power switch 122 is in the ON state. Diode D₁ 146 maybe reverse-biased when power switch 122 is in the ON state.

When power switch 122 is in the OFF state, power switch 122 may act asan open circuit and substantially prevent current through power switch122. When power switch 122 transitions from the ON state to the OFFstate, the polarity of secondary winding voltage V_(S) 144 reverses andenergy is transferred to output capacitor 110, which provides power toan electrical load connected to output terminals 104. Diode D₁ 146 mayallow charging of output capacitor 110 and the delivery of energy to aload after power switch 122 transitions to the OFF state. Although apassive rectification component (i.e., diode D₁ 146) is illustrated inFIG. 1, in other examples, power converter 100 may include a synchronousrectification switch (e.g., a MOSFET) that may be controlled bysecondary controller 120. In some examples, a synchronous rectificationswitch maybe integrated as a separate die inside integrated circuitpackage 124. Clamp circuit 148 is coupled to primary winding 114 ofenergy transfer element 112 to limit the maximum voltage on power switch122 when power switch 122 transitions between an ON state and an OFFstate.

Secondary controller 120 may sense an output quantity of power converter100 (e.g., output current IouT 109 and/or output voltage V_(OUT) 108).For example, secondary controller 120 of FIG. 1 senses feedback voltageVFB 132 at feedback terminal FB 128-7 (e.g., with respect to GNDterminal 128-6). In one example, feedback voltage VFB 132 sensed atfeedback terminal FB 128-7 is a scaled down voltage, e.g., by a resistordivider circuit, that is representative of output voltage V_(OUT) 108 ofpower converter 100. Although example secondary controller 120 of FIG. 1senses output voltage V_(OUT) 108 of power converter 100, it iscontemplated that, in some examples, secondary controller 120 may senseother output quantities, such as output current IouT 109 and/or acombination of output voltage V_(OUT) 108 and output current IouT 109 ofpower converter 100.

As described herein, primary controller 118 and secondary controller 120may operate to regulate an output quantity (e.g., output voltage V_(OUT)108 and/or output current IouT 109) of power converter 100. For example,primary controller 118 and secondary controller 120 may operate toregulate output voltage V_(OUT) 108 to a desired output voltage value inresponse to a sensed feedback voltage VFB 132. In general, incircumstances when output voltage V_(OUT) 108 drops to a value that isless than the desired output voltage value, primary controller 118 andsecondary controller 120 may operate to increase output voltage V_(OUT)108 until output voltage V_(OUT) 108 has reached the desired outputvoltage value. Although primary controller 118 and secondary controller120 may regulate output voltage V_(OUT) 108 in response to feedbackvoltage VFB 132, in some examples, primary controller 118 and secondarycontroller 120 may regulate output voltage V_(OUT) 108 and/or outputcurrent IouT 109 in response to sensed output current IouT 109 and/orfeedback voltage VFB 132.

Secondary controller 120 transmits request signal U_(REQ) 136 to primarycontroller 118. Primary controller 118 generates a switch drive signalU_(DRIVE) 138 that sets power switch 122 into the ON state in responseto receiving request signal U_(REQ) 136 from secondary controller 120.After primary controller 118 sets power switch 122 into the ON state inresponse to request signal U_(REQ) 136, primary controller 118determines when to set power switch 122 into the OFF state. After powerswitch 122 has been in the ON state for a period of time, primarycontroller 118 transitions power switch 122 from the ON state to the OFFstate at the time determined by primary controller 118, which may thenresult in a transfer of energy to the secondary side of power converter100. Power switch 122 may then be maintained in the OFF state by primarycontroller 118 until primary controller 118 receives another requestsignal U_(REQ) 136.

Secondary controller 120 may transmit multiple consecutive requestsignals U_(REQ) 136 over a period of time. Primary controller 118 mayset power switch 122 into the ON state in response to each of thetransmitted request signals U_(REQ) 136. As described herein, primarycontroller 118 and secondary controller 120 may be configured such thatthe amount of time between consecutive request signals U_(REQ) 136 isgreater than the amount of time during which power switch 122 is in theON state. Accordingly, in response to multiple consecutive requestsignals U_(REQ) 136, primary controller 118 may transition power switch122 into the ON state and back into the OFF state multiple consecutivetimes. As described herein, the timing between the transmission ofrequest signals U_(REQ) 136 may vary, e.g., depending on loadingconditions. Accordingly, the timing between transitions of power switch122 into the ON state may vary.

In some examples, request signal U_(REQ) 136 may be a pulse that istransmitted by secondary controller 120 and detected by primarycontroller 118. In these examples, secondary controller 120 may transmita plurality of consecutive pulses which may be separated by similar ordifferent amounts of time. Primary controller 118 may set power switch122 into the ON state in response to each pulse of the plurality ofconsecutive pulses.

After primary controller 118 sets power switch 122 into the ON state,primary controller 118 determines when to set power switch 122 into theOFF state. Primary controller 118 may set power switch 122 into the OFFstate in response to one or more “turn-off conditions.” Put another way,primary controller 118 may generate a switch drive signal U_(DRIVE) 138that sets power switch 122 into the OFF state when primary controller118 detects one or more turn-off conditions.

In one example, a turn-off condition may include an amount of currentthrough power switch 122. In this example, primary controller 118 maysense an amount of current through power switch 122 (i.e., switchcurrent) when power switch 122 is in the ON state. Primary controller118 may then set power switch 122 into an OFF state when the switchcurrent reaches a threshold current limit while power switch 122 is inthe ON state. In another example, a turn-off condition may include athreshold amount of time, referred to herein as a “conduction period.”In this example, primary controller 118 may be coupled to set powerswitch 122 into the ON state for a conduction period in response torequest signal U_(REQ) 136, and then set power switch 122 into the OFFstate after the conduction period has expired. Although turn-offconditions may include a threshold current limit and/or a conductionperiod, it is contemplated that primary controller 118 may set powerswitch 122 into the OFF state in response to other conditions. Theturn-off conditions (e.g., threshold current limit and/or the conductionperiod) may be fixed quantities in some examples. In other examples,primary controller 118 may adjust the turn-off conditions, e.g., inresponse to loading conditions detected by primary controller 118.

In summary, since primary controller 118 sets power switch 122 into theON state in response to request signal U_(REQ) 136 generated bysecondary controller 120, secondary controller 120 of the presentdisclosure may control when power switch 122 is set into the ON state.Additionally, since primary controller 118 determines when to transitionpower switch 122 from the ON state into the OFF state, primarycontroller 118 controls how long power switch 122 remains in the ONstate. As described hereinafter with respect to FIG. 2, secondarycontroller 120 may include a timing circuit (e.g., timing circuit 258)that controls how often request signals U_(REQ) 136 are sent to primarycontroller 118. In other words, secondary controller 120 may control howoften power switch 122 may be set into the ON state by primarycontroller 118. Accordingly, secondary controller 120 may control therate (e.g., the max rate) at which power switch 122 is set into the ONstate.

Operation of example circuits included in primary controller 118 andsecondary controller 120 is now described in greater detail with respectto FIG. 2. FIG. 2 shows an example integrated circuit package 224 thatincludes a power switch 222 (e.g., power MOSFET 222), an example primarycontroller 218, and an example secondary controller 220. Circuitsexternal to integrated circuit package 224 may electrically couple topackage terminals D 226-1, S 226-2, PBP 226-3, FWD 226-4, BP 226-5, GND226-6, and FB 226-7 (collectively “package terminals 226”) of integratedcircuit package 224.

Package terminals 226 may connect to terminals D 228-1, S 228-2, PBP228-3, FWD 228-4, BP 228-5, GND 228-6, and FB 228-7 of power switch 222,primary controller 218, and secondary controller 220 included on theinside of integrated circuit package 224. Package terminals 226 may beconnected to a power converter in a similar manner as illustrated inFIG. 1. Accordingly, the description of integrated circuit package 224may hereinafter reference the components of power converter 100 of FIG.1.

Primary controller 218 includes a primary switch control circuit 250, acurrent sense circuit 252, and memory circuit 254. Secondary controller220 includes a secondary switch control circuit 256 and a timing circuit258. Primary controller 218 and secondary controller 220 may includeadditional circuits that are not illustrated in FIG. 2. For example,primary controller 218 may include circuits that couple to terminal PBP228-3 to provide power to primary controller 218. Secondary controller220 may include circuits that charge bypass capacitor 130. For example,secondary controller 220 may include circuits that couple to forwardterminal FWD 228-4 and bypass terminal BP 228-5 to charge bypasscapacitor 130 from forward terminal FWD 228-4. Ground terminal GND 228-6may be the output return for circuits of secondary controller 220.

In some examples, secondary controller 220 may include additionalterminals that are not illustrated in FIGS. 1-2, such as a terminal thatis coupled to output terminal 104-1. In these examples, secondarycontroller 220 may include circuits that charge bypass capacitor 130from output terminal 104-1. For example, secondary controller 220 maycharge bypass capacitor 130 from forward terminal FWD 228-4 and/or theadditional terminal coupled to output terminal 104-1. Charging bypasscapacitor 130 from output terminal 104-1 during operation may be moreefficient than charging bypass capacitor 130 from forward terminal FWD228-4.

Primary switch control circuit 250 generates switch drive signalU_(DRIVE) 238 that sets the state of power switch 222. Primary switchcontrol circuit 250 may maintain power switch 222 in the OFF state untilprimary switch control circuit 250 receives request signal U_(REQ) 236from secondary switch control circuit 256. Primary switch controlcircuit 250 transitions power switch 222 from the OFF state to the ONstate in response to receiving request signal U_(REQ) 236.

After setting power switch 222 into the ON state, primary switch controlcircuit 250 determines when to set power switch 222 into the OFF state.For example, primary switch control circuit 250 may set power switch 222into the OFF state in response to detection of one or more turn-offconditions. In one example, a turn-off condition may be an amount ofswitch current I_(SWITCH) 260 through power switch 222 (i.e., athreshold current limit). In other examples, a turn-off condition may bea threshold amount of time (i.e., a conduction period).

Memory circuit 254 may store the one or more turn-off conditions. Forexample, memory circuit 254 may store the threshold current limit and/orthe conduction period. In some examples, turn-off conditions may havefixed values. In other examples described herein, primary switch controlcircuit 250 may adjust the values of the turn-off conditions. Forexample, primary switch control circuit 250 may determine loadingconditions of power converter 100 and adjust the turn-off conditions inmemory circuit 254 in response to determined loading conditions.

In some examples, memory circuit 254 may include circuits that count anumber of request signals U_(REQ) 236 received from secondary switchcontrol circuit 256. The number of request signals U_(REQ) 236 receivedover a period of time may be indicative of loading conditions at theoutput of power converter 100. In these examples, primary switch controlcircuit 250 may determine when to set power switch 222 into the OFFstate based on the count maintained by memory circuit 254. In otherexamples, memory circuit 254 may include a capacitor that may be chargedwhile power switch 222 is in the ON state and discharged while powerswitch 222 is in the OFF state. In these examples, the voltage acrossthe capacitor of memory circuit 254 may indicate the amount of timepower switch 222 has been in the ON state over a period of time, whichmay indicate loading conditions at the output of power converter 100.Primary switch control circuit 250 may determine when to set powerswitch 222 into the OFF state in response to the voltage across thecapacitor of memory circuit 254.

Current sense circuit 252 may sense an amount of switch currentI_(SWITCH) 260 through power switch 222 when power switch 222 is in theON state. Primary switch control circuit 250 may determine when theamount of current through power switch 222 has reached the thresholdcurrent limit based on the amount of current sensed by current sensecircuit 252. In examples where a threshold current limit is a turn-offcondition, primary switch control circuit 250 sets power switch 222 intothe OFF state in response to a determination, that the current throughpower switch 222 has reached the threshold current limit.

In examples where primary switch control circuit 250 uses a thresholdamount of time as a turn-off condition (i.e., a conduction period),primary switch control circuit 250 may determine the amount of time thathas elapsed since primary switch control circuit. 250 has set powerswitch 222 into the ON state. In these examples, primary switch controlcircuit 250 sets power switch 222 into the OFF state after power switch222 has been in the ON state for one conduction period.

In some examples, primary switch control circuit 250 may set powerswitch 222 into the OFF state in response to a threshold current limitwithout monitoring the amount of time power switch 222 has been in theON state. In other examples, primary switch control circuit 250 may notmonitor the amount of current through power switch 222, but instead,primary switch control circuit 250 may set power switch 222 into the OFFstate after power switch 222 has been in the ON state for one conductionperiod. In still other examples, primary switch control circuit 250 mayset power switch 222 into the OFF state in response to more than oneturn-off condition. For example, primary switch control circuit 250 mayset power switch 222 into the OFF state when a conduction period haspassed or when the threshold current limit is reached, whichever occursfirst. Although turn-off conditions may include at least one of athreshold time limit (i.e., a conduction period) and a threshold currentlimit, it is contemplated that other turn-off conditions may be used byprimary switch control circuit 250 to determine when to set power switch222 into the OFF state.

Timing circuit 258 and secondary switch control circuit 256 control whenrequest signals U_(REQ) 236 are sent to primary controller 218, which inturn controls when power switch 222 is set into an ON state. Asdescribed herein, secondary switch control circuit 256 may generaterequest signal U_(REQ) 236 in response to a state of timing circuit 258and a sensed output quantity of power converter 100 (e.g., feedbackvoltage VFB 132).

Timing circuit 258 may be set in one of a first state and a secondstate. In general, timing circuit 258 may be in the first state untiltiming circuit 258 is triggered by secondary switch control circuit 256to transition to the second state. As described herein, when timingcircuit 258 is in the second state, secondary switch control circuit 256may decide to withhold transmission of request signal U_(REQ) 236. Insome examples, timing circuit 258 may be implemented using an oscillatorcircuit (e.g., an RC oscillator circuit). Although timing circuit 258 isillustrated and described herein as operating in one of a first stateand a second state, it is contemplated that the functionality associatedwith timing circuit 258 may be implemented using a variety of differentcircuit components.

When timing circuit 258 is triggered by secondary switch control circuit256, timing circuit 258 may transition from the first state to thesecond state and remain in the second state for a period of time,referred to herein as a “holding period.” After timing circuit 258 hasbeen in the second state for a holding period, timing circuit 258 maytransition back to the first state. Timing circuit 258 may stay in thefirst state until triggered by secondary switch control circuit 256 toreturn to the second state for a holding period. In another example,timing circuit 258 may include an oscillator (not shown) thatperiodically sets timing circuit 258 to the first state, and is thenautomatically set back to the second state independent of output voltageV_(OUT) 108. In other words, the timing circuit 258 is periodically setto a first state at the beginning of a switching cycle period and isthen automatically set back to the second state. In this manner,transmission of request signal U_(REQ) 236 may only occur at set times,based on the frequency of the oscillator (not shown) in timing circuit258.

Secondary switch control circuit 256 receives feedback voltage VFB 132that is representative of output voltage V_(OUT) 108. Secondary switchcontrol circuit 256 determines when output voltage V_(OUT) 108 is lessthan a desired output voltage value based on the value of feedbackvoltage VFB 132. Secondary switch control circuit 256 may also determinethe state of timing circuit 258. For example, secondary switch controlcircuit 256 may determine whether timing circuit 258 is in the firststate or the second state.

Secondary switch control circuit 256 transmits request signal U_(REQ)236 and triggers timing circuit 258 when secondary switch controlcircuit 256 determines that timing circuit 258 is in the first state andoutput voltage V_(OUT) 108 is less than the desired output voltagevalue. For example, secondary switch control circuit 256 may transmitrequest signal U_(REQ) 236 to primary controller 218 and also generate atrigger signal on the secondary side that triggers timing circuit 258 toenter the second state. Secondary switch control circuit 256 maytransmit request signal U_(REQ) 236 and also trigger timing circuit 258at approximately the same time. Since primary controller 218 sets powerswitch 222 into the ON state in response to request signal U_(REQ) 236,timing circuit 258 may transition to the second state at approximatelythe same time as power switch 222 is set into the ON state by primarycontroller 218.

In circumstances when timing circuit 258 is in the second state oroutput voltage V_(OUT) 108 is greater than the desired output voltagevalue, secondary switch control circuit 256 may decide to withholdtransmission of request signal U_(REQ) 236. In one circumstance, whenoutput voltage V_(OUT) 108 is greater than the desired output voltagevalue and timing circuit 258 is in the first state, secondary switchcontrol circuit 256 may withhold transmission of request signal U_(REQ)236 until output voltage V_(OUT) 108 drops to a value that is less thanthe desired output voltage value. In this example, secondary switchcontrol circuit 256 may transmit request signal U_(REQ) 236 and triggertiming circuit 258 to enter the second state when the output voltageV_(OUT) 108 drops to less than the desired output voltage value. Inanother circumstance, when output voltage V_(OUT) 108 is less than thedesired output voltage value and timing circuit 258 is in the secondstate, secondary switch control circuit 256 may decide to withholdtransmission of request signal U_(REQ) 236 until timing circuit 258transitions to the first state. In this example, secondary switchcontrol circuit 256 may transmit request signal U_(REQ) 236 and triggertiming circuit 258 back to the second state in response to timingcircuit 258 entering the first state, assuming that output voltageV_(OUT) 108 is still less than the desired output voltage value whentiming circuit 258 enters the first state.

Secondary switch control circuit 256 controls the rate at which requestsignals U_(REQ) 236 are transmitted to primary controller 218. Putanother way, secondary switch control circuit 256 controls how manyrequest signals U_(REQ) 236 are transmitted to primary controller 218during a period of time. As described herein, multiple consecutiverequest signals U_(REQ) 236 may be separated from one another by varyingamounts of time, depending on when secondary switch control circuit 256determines when to transmit request signals U_(REQ) 236. Accordingly,secondary switch control circuit 256 may control the rate at which powerswitch 222 is set into the ON state since primary switch control circuit250 sets power switch 222 into the ON state in response to each requestsignal U_(REQ) 236.

Secondary switch control circuit 256 may control the rate at whichrequest signals U_(REQ) 236 are sent to primary switch control circuit250 in response to an amount of loading at output terminals 104 of powerconverter 100. For example, secondary switch control circuit 256 maytend to transmit request signals U_(REQ) 236 at a greater rate duringheavier loading since output voltage V_(OUT) 108 may tend to drop belowthe desired output voltage value faster during heavier loading. Inexamples where the loading at output terminals 104 decreases, secondaryswitch control circuit 256 may tend to transmit request signals U_(REQ)236 at a lower rate than when heavier loading is present at outputterminals 104.

When timing circuit 258 is triggered to enter the second state bysecondary switch control circuit 256, timing circuit 258 may remain inthe second state for a holding period until timing circuit 258transitions back to the first state. The holding period of timingcircuit 258 may set a maximum rate at which request signals U_(REQ) 236may be transmitted since secondary switch control circuit 256 withholdstransmission of a request signal U_(REQ) 236 while timing circuit 258 isin the second state. Accordingly, the holding period of timing circuit258 may set a maximum rate at which power switch 222 may be set into theON state. Put another way, the holding period may be approximately equalto the minimum time between two consecutive request signals U_(REQ) 236,or two consecutive transitions of power switch 222 into the ON state. Incircumstances where output voltage V_(OUT) 108 is below the desiredoutput voltage value when timing circuit 258 transitions from the secondstate to the first state, request signals U_(REQ) 236 may be spaced byapproximately one holding period. Such circumstances may arise duringheavy loading, which may cause secondary switch control circuit 256 totransmit request signals U_(REQ) 236 that are each separated byapproximately one holding period.

The holding period of timing circuit 258 may be set to a value thatallows a sufficient amount of time for energy to be transferred from theprimary side of power converter 100 to the secondary side. For example,the holding period may be set to a value that allows for energy transferto the secondary side after primary switch control circuit 250 hastransitioned power switch 222 from the ON state to the OFF state. Sinceprimary switch control circuit 250 determines how long power switch 222remains in the ON state, the holding period of timing circuit 258 andthe turn-off conditions of power switch 222 may be selected such that asufficient amount of energy is transferred after power switch 222 is setinto the OFF state. For example, the holding period may be selected tobe greater than an expected amount of time power switch 222 will remainin the ON state plus an expected amount of time that allows forsufficient energy transfer to the secondary side after power switch 222is set into the OFF state.

Primary switch control circuit 250 may adjust the turn-off conditions(e.g., threshold current limit and/or the conduction period) in responseto loading conditions of the power converter 100. In some examples,primary switch control circuit 250 may determine loading conditionsbased on how long power switch 222 is in the ON state during a period oftime. In general, power switch 222 may be maintained in the ON statemore often during heavier loading conditions. In these examples, primaryswitch control circuit 250 may determine that heavier loading conditionsexist when power switch 222 is maintained in the ON state for a greateramount of time during a given time period. Similarly, primary switchcontrol circuit 250 may determine that lighter loading conditions existwhen power switch 222 is maintained in the ON state for a lesser amountof time during the given time period.

In other examples, primary switch control circuit 250 may determineloading conditions based on how many request signals U_(REQ) 236 arereceived by primary switch control circuit 250 over a period of time. Ingeneral, primary switch control circuit 250 may receive a greater numberof request signals U_(REQ) 236 during heavier loading conditions. Inthese examples, primary switch control circuit 250 may determine thatheavier loading conditions exist when a greater number of requestsignals U_(REQ) 236 are received during a given time period. Similarly,primary switch control circuit 250 may determine that lighter loadingconditions exist when a lesser number of request signals U_(REQ) 236 arereceived during the given time period.

In other examples, primary switch control circuit 250 may determineloading conditions based on the amount of time between consecutiverequest signals U_(REQ) 236 received by primary switch control circuit250. In general, the amount of time between consecutive request signalsU_(REQ) 236 may be less during heavier loading conditions than duringlighter loading conditions. In these examples, primary switch controlcircuit 250 may determine that heavier loading conditions exist when theamount of time between consecutive request signals U_(REQ) 236 is lessthan a threshold amount of time. Similarly, primary switch controlcircuit 250 may determine that lighter loading conditions exist when theamount of time between consecutive request signals U_(REQ) 236 isgreater than the threshold amount of time.

In examples where primary switch control circuit 250 detects an increasein loading, primary switch control circuit 250 may adjust the turn-offconditions in memory circuit 254 such that power switch 222 ismaintained in the ON state for a greater amount of time. Maintainingpower switch 222 in the ON state for a greater amount of time may resultin more energy transfer to output terminals 104 during increasedloading. In examples where a threshold current limit is used as aturn-off condition, primary switch control circuit 250 may increase thevalue of the threshold current limit so that power switch 222 may remainin the ON state for a greater amount of time. In examples where athreshold time limit (i.e., conduction period) is used as a turn-offcondition, primary switch control circuit 250 may increase the durationof the conduction period so that power switch 222 may remain in the ONstate for a greater amount of time.

In examples where primary switch control circuit 250 detects a decreasein loading, primary switch control circuit 250 may adjust the turn-offconditions in memory circuit 254 such that power switch 222 ismaintained in the ON state for a lesser amount of time. Maintainingpower switch 222 in the ON state for a lesser amount of time may resultin less energy transfer to output terminals 104 during decreasedloading. In examples where a threshold current limit is used as aturn-off condition, primary switch control circuit 250 may decrease thevalue of the threshold current limit so that power switch 222 may remainin the ON state for a lesser amount of time. In examples where athreshold time limit (i.e., conduction period) is used as a turn-offcondition, primary switch control circuit 250 may decrease the durationof the conduction period so that power switch 222 may remain in the ONstate for a lesser amount of time.

In some examples, primary switch control circuit 250 may be coupled toignore a received request signal U_(REQ) 236. In other words, in someexamples, primary switch control circuit 250 may refrain from switchingpower switch 222 into the ON state when a request signal U_(REQ) 236 isreceived. For example, primary switch control circuit 250 may ignore areceived request signal during abnormal or fault conditions. Abnormal orfault conditions may include circumstances where switch currentI_(SWITCH) 260 fails to reach a threshold current limit within anexpected period of time, e.g., due to an abnormally low input voltageV_(IN) 106. Other abnormal or fault conditions may also includecircumstances in which noise induced in the communication link mayappear to be a request signal transmitted from secondary switch controlcircuit 256. Such noise may be induced in the communication link byswitching of power switch 222, or operation of other circuits of primarycontroller 218, secondary controller 220. In other examples, noise mayalso originate from other electronic equipment in the vicinity. Underthese conditions, primary switch control circuit 250 may receive arequest signal U_(REQ) 236 when power switch 222 is in the ON state orwhen power switch 222 has just transitioned to the OFF state. Primaryswitch control circuit 250 may ignore a received request signal U_(REQ)236 if the request signal U_(REQ) 236 is received while primary switchcontrol circuit 250 has power switch 222 set in the ON state. Similarly,primary switch control circuit 250 may ignore a received request signalU_(REQ) 236 if the request signal U_(REQ) 236 is received immediatelyafter (e.g., within a threshold amount of time) primary switch controlcircuit 250 transitions power switch 222 to the OFF state. In general,primary controller 218 and secondary controller 220 are configured suchthat request signals U_(REQ) 236 are not sent close enough together thata request signal is received while power switch 222 is in the ON state.

Primary switch control circuit 250 may also ignore a request signalU_(REQ) 236 under other fault conditions. In one example, primary switchcontrol circuit 250 may determine when a component (e.g., power switch222) is overheated, or susceptible to overheating. In response to such adetermination, primary switch control circuit 250 may ignore a requestsignal U_(REQ) 236 and refrain from setting power switch 222 into the ONstate so that power switch 222, or other component, does not becomethermally damaged if normal switching were to be maintained. In theseexamples, primary switch control circuit 250 may include circuits thatsense temperature, or a temperature sensor that is readable by primaryswitch control circuit 250 may be included external to primarycontroller 218. In another example, primary switch control circuit 250may detect when input voltage exceeds an input voltage threshold thatmay damage power switch 222. In these examples, primary switch controlcircuit 250 may ignore a received request signal U_(REQ) 236 whenprimary switch control circuit 250 determines that an input voltage maydamage power switch 222 if normal switching is maintained.

FIG. 3 is a flow diagram that describes operation of secondarycontroller 220. Prior to the start of method 300, it may be assumed thattiming circuit 258 is in the first state and that output voltage V_(OUT)108 has dropped to a value that is less than the desired output voltagevalue. In block 302, secondary switch control circuit 256 decides totransmit request signal U_(REQ) 236 in response to determining thattiming circuit 258 is in the first state and that output voltage V_(OUT)108 is less than the desired output voltage value. In block 304,secondary switch control circuit 256 triggers timing circuit 258 totransition from the first state to the second state. For example,secondary switch control circuit 256 may trigger timing circuit 258 atapproximately the same time that secondary switch control circuit 256transmits request signal U_(REQ) 236.

After timing circuit 258 is triggered by secondary switch controlcircuit 256, timing circuit 258 may remain in the second state for aholding period. In block 306, secondary switch control circuit 256determines whether the holding period has expired. Secondary switchcontrol circuit 256 may determine that the holding period has notexpired if timing circuit 258 is in the second state. Secondary switchcontrol circuit 256 may determine that the holding period has expiredwhen timing circuit 258 is in the first state. Accordingly, if secondaryswitch control circuit 256 determines that timing circuit 258 is in thesecond state, secondary switch control circuit 256 may continue tomonitor the state of timing circuit 258 in block 306.

After the holding period has expired, timing circuit 258 transitions tothe first state in block 308. Upon transitioning to the first state,secondary switch control circuit 256 may determine that timing circuit258 is in the first state in block 308. In block 310, secondary switchcontrol circuit 256 senses an output quantity of power converter 100,such as output voltage V_(OUT) 108 and/or output current IouT 109. Ifthe sensed output quantity is not less than a desired output quantity(e.g., a desired output voltage value) in block 312, then secondaryswitch control circuit 256 may continue to sense the output quantity ofpower converter 100 in block 310. Since timing circuit 258 is in thefirst state, if secondary switch control circuit 256 determines that thesensed output quantity is less than a desired output quantity in block312, secondary switch control circuit 256 may transmit request signalU_(REQ) 236 in block 302.

FIG. 4 is a flow diagram that describes operation of primary controller218. Prior to the start of method 400, it may be assumed that primaryswitch control circuit 250 is maintaining power switch 222 in the OFFstate. In block 402, primary switch control circuit 250 determineswhether request signal U_(REQ) 236 has been received. If primary switchcontrol circuit 250 does not detect request signal U_(REQ) 236, primaryswitch control circuit 250 may continue waiting for request signalU_(REQ) 236 in block 402.

If primary switch control circuit 250 receives request signal U_(REQ)236, primary switch control circuit 250 may adjust or maintain theturn-off conditions in block 404. For example, primary switch controlcircuit 250 may adjust the turn-off conditions in memory circuit 254 inresponse to changing loading conditions. In other examples, primaryswitch control circuit 250 may maintain the turn-off conditions inmemory circuit 254 when loading conditions have not changed. Althoughprimary switch control circuit 250 may adjust/maintain the turn-offconditions after request signal U_(REQ) 236 is received in block 402, itis contemplated that primary switch control circuit 250 mayadjust/maintain the turn-off conditions at other times during operation(e.g., after block 406). Although primary switch control circuit 250 mayadjust the turn-off conditions in some examples, in other examples, theturn-off conditions may be fixed. In these examples, primary switchcontrol circuit 250 may not adjust the turn-off conditions, and block404 may be removed from method 400.

Primary switch control circuit 250 sets power switch 222 into the ONstate in response to the received request signal U_(REQ) 236 in block406. In block 408, primary switch control circuit 250 determines whethera turn-off condition is detected. As described herein, a turn-offcondition may include, but is not limited to, a threshold current limitand/or a threshold amount of time. If primary switch control circuit 250does not detect a turn-off condition, primary switch control circuit 250may maintain power switch 222 in the ON state and continue to monitorfor turn-off conditions. If primary switch control circuit 250 detects aturn-off condition, primary switch control circuit 250 sets power switch222 into the OFF state in block 410.

FIG. 5 is a flow diagram that describes operation of primary controller218 and secondary controller 220. FIG. 5 describes transmission of asingle request signal U_(REQ) 236 and response of primary switch controlcircuit 250 to the single request signal U_(REQ) 236. Although operationof primary controller 218 and secondary controller 220 are describedwith respect to transmission of a single request signal U_(REQ) 236,method 500 may be repeated by primary controller 218 and secondarycontroller 220 during operation. For example, method 500 may loop backfrom block 520 to block 502 instead of ending after block 520.

In block 502, timing circuit 258 is in the first state. In block 504,secondary switch control circuit 256 senses an output quantity of powerconverter 100, such as output voltage V_(OUT) 108 and/or output currentIouT 109. If the sensed output quantity is not less than a desiredoutput quantity, then secondary switch control circuit 256 may continueto monitor the output quantity of power converter 100. If secondaryswitch control circuit 256 determines that the sensed output quantity isless than a desired output quantity in block 506, secondary switchcontrol circuit 256 transmits request signal U_(REQ) 236 and triggerstiming circuit 258 in block 508.

In block 510, primary switch control circuit 250 receives request signalU_(REQ) 236. Primary switch control circuit 250 sets power switch 222into the ON state in block 512 in response to receiving request signalU_(REQ) 236. In block 514, primary switch control circuit 250 may adjustor maintain the turn-off conditions. In block 516, primary switchcontrol circuit 250 determines whether a turn-off condition is detected.If primary switch control circuit 250 does not detect a turn-offcondition, primary switch control circuit 250 may maintain power switch222 in the ON state and continue to monitor for turn-off conditions. Ifprimary switch control circuit 250 detects a turn-off condition, primaryswitch control circuit 250 sets power switch 222 into the OFF state inblock 518. As described above, the holding period may be selected to begreater than an expected amount of time power switch 222 will remain inthe ON state plus an expected amount of time that allows for sufficientenergy transfer to the secondary side after power switch 222 is set intothe OFF state. Accordingly, in block 520, the holding period of timingcircuit 258 ends and timing circuit 258 transitions to the first state.

FIG. 6 shows waveforms illustrating operation of primary controller 218and secondary controller 220 under varying load conditions.Specifically, FIG. 6 shows switch current I_(SWITCH) 260 and outputvoltage V_(OUT) 108 waveforms along with corresponding timing circuitstates (T.C. STATE), switch drive signals U_(DRIVE) 238, and requestsignals U_(REQ) 236. The timing circuit state waveform of FIG. 6 ismeant to graphically illustrate the states of timing circuit 258.Although the timing circuit waveform of FIG. 6 illustrates the first andsecond states as corresponding to low and high values, respectively,such a representation of the timing circuit states is provided forexplanation purposes. Accordingly, the timing circuit state waveform mayor may not represent digital/analog values (e.g., voltages) associatedwith an implementation of timing circuit 258.

The left half of FIG. 6 illustrates operation of primary controller 218and secondary controller 220 under heavier load conditions. The righthalf of FIG. 6 illustrates operation of primary controller 218 andsecondary controller 220 under lighter load conditions. A time gap 661between heavier and lighter load conditions is illustrated for each ofthe signals in FIG. 6.

At time zero, output voltage V_(OUT) 108 is greater than the desiredoutput voltage value (indicated by dashed line 662). Additionally,timing circuit 258 is in the first state. At 663, output voltage V_(OUT)108 drops to a value that is less than the desired output voltage value.Accordingly, at 663, secondary switch control circuit 256 transmits arequest signal 664 and triggers timing circuit 258 to enter the secondstate. In response to receiving request signal 664, primary switchcontrol circuit 250 generates switch drive signal U_(DRIVE) 238 thatsets power switch 222 into the ON state.

Primary switch control circuit 250 may then determine when to set powerswitch 222 into the OFF state. In FIG. 6, it may be assumed that primaryswitch control circuit 250 uses a threshold current limit as a turn-offcondition. Additionally, it may be assumed that the threshold currentlimit is adjustable based on loading conditions. As illustrated at 665,primary switch control circuit 250 may adjust the threshold currentlimit to three different values (I_(LIM1), I_(LIM2), and I_(LIM3)),depending on loading conditions.

As indicated at 666, switch current I_(SWITCH) 260 increases while powerswitch 222 is in the ON state. Primary switch control circuit 250 maysense switch current I_(SWITCH) 260 and set power switch 222 into theOFF state when switch current I_(SWITCH) 260 reaches a threshold currentlimit I_(LIM2). Energy is transferred to the secondary side after powerswitch 222 is set into the OFF state. Although energy is transferred tothe secondary side of power converter 100, output voltage V_(OUT) 108remains at a value that is less than the desired output voltage value.

After power switch 222 is set in the OFF state at 667, timing circuit258 remains in the second state until expiration of the holding periodat 668. Output voltage V_(OUT) 108 is less than the desired outputvoltage value upon expiration of the holding period. Accordingly, uponexpiration of the holding period, secondary switch control circuit 256may trigger timing circuit 258 to return to the second state and alsotransmit request signal 669. The delay period 670 between twoconsecutive holding periods may represent a time period during whichsecondary switch control circuit 256 determines the state of timingcircuit 258 and determines whether output voltage V_(OUT) 108 is lessthan the desired output voltage value. T_(MIN) in FIG. 6 may representthe minimum time between two consecutive request signals. In otherwords, T_(MIN) may represent the minimum amount of time between twoconsecutive transitions of the power switch into the ON state.

Primary switch control circuit 250 may set power switch 222 into the ONstate a second time at 671. At 672, primary switch control circuit 250sets power switch 222 into the OFF state when switch current I_(SWITCH)260 reaches the threshold current limit I_(LIM2). Although energy istransferred to the secondary side after setting power switch 222 intothe OFF state, output voltage V_(OUT) 108 remains at a value that isless than the desired output voltage value.

After setting power switch 222 into the OFF state at 672, primary switchcontrol circuit 250 adjusts the turn-off conditions (i.e., the thresholdcurrent limit). For example, primary switch control circuit 250increases the threshold current limit to a value of I_(LIM3) in order totransfer more energy to the secondary side during future transitions ofpower switch 222 from the ON state to the OFF state. As illustrated at673, primary switch control circuit 250 sets power switch 222 into theOFF state when switch current I_(SWITCH) 260 reaches I_(LIM3), whichresults in output voltage V_(OUT) 108 increasing to a value that isgreater than the desired output voltage value at 674. It is contemplatedthat the threshold current limit value I_(LIM) could be varied for eachON time of power switch 222.

After a period of time, illustrated by time gap 661, power converter 100is experiencing lighter loading conditions. During lighter loadingconditions, primary side control circuit 250 may decrease the thresholdcurrent limit to a lesser value of I_(LIM1). Accordingly, primary switchcontrol circuit 250 may set power switch 222 into the OFF state whenswitch current I_(SWITCH) 260 reaches I_(LIM1) during lighter loadingconditions.

FIGS. 7A-7B show example communication links through which requestsignal U_(REQ) 236 may be transmitted. The communication linksillustrated in FIGS. 7A-7B may be included in integrated circuit package224 of the present disclosure. FIG. 7A shows an example magneticallycoupled communication link included in integrated circuit package 224.Integrated circuit package 224 includes a primary conductive loop 775and a secondary conductive loop 776 that are galvanically isolated fromone another. In some examples, primary conductive loop 775 and secondaryconductive loop 776 may be isolated conductors of the lead frame ofintegrated circuit package 224. By integrating primary and secondaryconductive loops 775, 776 in the lead frame of integrated circuitpackage 224, the communication link between the primary and secondarysides of power converter 100 may be added to integrated circuit package224 with less cost.

Primary conductive loop 775 may be coupled to primary switch controlcircuit 250. Secondary conductive loop 776 may be coupled to secondaryswitch control circuit 256. Although primary and secondary conductiveloops 775, 776 are galvanically isolated from one another, primary andsecondary conductive loops 775, 776 may be magnetically coupled suchthat a change in current through secondary conductive loop 776 mayinduce a voltage/current in primary conductive loop 775. Secondaryswitch control circuit 256 may transmit request signal U_(REQ) 236 toprimary switch control circuit 250 by inducing a change in currentthrough secondary conductive loop 776. Primary switch control circuit250 may detect request signal U_(REQ) 236 by detecting an inducedvoltage and/or current in primary conductive loop 775.

FIG. 7B shows an example optically coupled communication link includedin integrated circuit package 224. Integrated circuit package 224includes an optical transmitter 777 (e.g., a light-emitting diode) andan optical receiver 778 (e.g., a phototransistor) that are galvanicallyisolated from one another. Optical receiver 778 may be coupled toprimary switch control circuit 250. Optical transmitter 777 may becoupled to secondary switch control circuit 256. Although opticaltransmitter 777 and optical receiver 778 are galvanically isolated fromone another, optical transmitter 777 and optical receiver 778 may forman optical communication link. For example, optical transmitter 777 mayemit light that is detected by optical receiver 778. Secondary switchcontrol circuit 256 may transmit request signal U_(REQ) 236 to primaryswitch control circuit 250 by energizing optical transmitter 777 to emitlight. Primary switch control circuit 250 may detect request signalU_(REQ) 236 by detecting an induced voltage and/or current generated byoptical receiver 778 in response to the light emitted by opticaltransmitter 777. It is contemplated that communication link technologiesother than magnetically and optically coupled communication links may beused. For example, a capacitive coupling may be used as a communicationlink between primary controller 218 and secondary controller 220.

FIG. 8 shows an example non-isolated power converter 880 that includes aprimary controller 881 and a secondary controller 882 of the presentdisclosure. Although primary controller 881 and secondary controller 882are illustrated as included in a non-isolated power converter 880 havinga buck converter topology, it is contemplated that primary controller881 and secondary controller 882 may be included in non-isolated powersupplies having other topologies. As described hereinafter, primarycontroller 881 and secondary controller 882 may operate in a similarmanner as primary controller 218 and secondary controller 220 describedabove.

Power converter 880 includes input terminals 883-1, 883-2 (collectively“input terminals 883”) and output terminals 884-1, 884-2 (collectively“output terminals 884”). Input terminals 883 are coupled to receive aninput voltage V_(IN) 885, which may be a rectified and filtered acvoltage. Output terminals 884 provide an output voltage V_(OUT) 886 to aload (not shown).

Power converter 880 includes input capacitor 887, output capacitor 888,inductor 889, diode 890, and power switch 891. As illustrated in FIG. 8,power switch 891, diode 890, and inductor 889 are coupled to operate asa buck converter circuit. Secondary controller 882 may be coupled toreceive operating power from node 892 and/or bypass capacitor 893.Secondary controller 882 may also be coupled to output terminals 884 viaa feedback circuit 896 that allows secondary controller 882 to sense anoutput quantity of power converter 880 (e.g., output voltage V_(OUT)886).

Primary controller 881 and secondary controller 882 are galvanicallyisolated from one another. Although primary controller 881 and secondarycontroller 882 are galvanically isolated from one another, secondarycontroller 882 may transmit a request signal U_(REQ) 894 to primarycontroller 881 via a communication link (e.g., a magnetically,capacitively, or optically coupled communication link). Primarycontroller 881 may generate switch drive signal U_(DRIVE) 895 to setpower switch 891 into an ON state in response to a request signalU_(REQ) 894 received from secondary controller 882.

Primary controller 881 and secondary controller 882 may operate toregulate an output quantity of power converter 880 in a similar manneras described above with respect to primary controller 218 and secondarycontroller 220. For example, secondary controller 882 may includecircuits (e.g., a secondary switch control circuit and a timing circuit)that control generation of request signals U_(REQ) 894 to control howoften power switch 891 is set into the ON state by primary controller881. After each time primary controller 881 sets power switch 891 intothe ON state, primary controller 881 determines when to set power switch891 into the OFF state, e.g., in response to one or more turn-offconditions.

As described above, secondary switch control circuit 256 transmitsrequest signal U_(REQ) 236 and triggers timing circuit 258 whensecondary switch control circuit 256 determines that timing circuit 258is in the first state and output voltage V_(OUT) 108 is less than thedesired output voltage value. Although secondary switch control circuit256 is described above as triggering timing circuit 258 when outputvoltage V_(OUT) 108 is less than the desired output voltage value andtiming circuit 258 is in the first state, timing circuit 258 may betriggered in a different manner in some implementations of a secondarycontroller. An alternative implementation of a secondary controller inwhich timing circuit 258 is triggered in a different manner is describedhereinafter with respect to FIG. 9.

FIG. 9 shows alternate waveforms illustrating operation of primarycontroller 218 and secondary controller 220. Specifically, FIG. 9 showsan output voltage V_(OUT) 108 waveform along with corresponding timingcircuit states (T.C. STATE), switch drive signals U_(DRIVE) 238, andrequest signals U_(REQ) 236. In FIG. 9, timing circuit 258 includes anoscillator that periodically sets timing circuit 258 to a first statebased on a set frequency. In this example, timing circuit 258 may beautomatically set to the second state after it is briefly set to thefirst state in response to an oscillator internal to timing circuit 258.In this example, the state of timing circuit 258 may be determined inresponse to an oscillator (not shown) and may be independent ofsecondary switch control circuit 256.

At time zero, output voltage V_(OUT) 108 is greater than the desiredoutput voltage value (indicated by dashed line 1002). At 1004, outputvoltage V_(OUT) 108 drops to a value that is less than the desiredoutput voltage value. When timing circuit is set to the first state,secondary switch control circuit 256 transmits a request signal U_(REQ)1006 since timing circuit 258 is in the first state when output voltageV_(OUT) 108 is less than the desired output voltage value. In responseto receiving request signal U_(REQ) 1006, primary switch control circuit250 sets power switch 222 into the ON state and then the OFF state(e.g., in response to a turn-off condition), as indicated at 1008. Theenergy transferred to the secondary side of power converter 100 causesoutput voltage V_(OUT) 108 to increase to a value that is greater thanthe desired output voltage value.

After a period of time, output voltage V_(OUT) 108 again drops to avalue that is less than the desired output voltage value at 1010.Secondary switch control circuit 220 may withhold transmission ofrequest signal U_(REQ) 236 at 1010 because timing circuit 258 is in thesecond state. Secondary switch control circuit 220 may transmit arequest signal U_(REQ) 1012 when timing circuit 258 transitions to thefirst state at 1014. In response to receiving request signal U_(REQ)1012, primary switch control circuit 250 sets power switch 222 into theON state and then the OFF state (e.g., in response to a turn-offcondition), as indicated at 1016. The energy transferred to thesecondary side of power converter 100 causes output voltage V_(OUT) 108to increase. At the next time, timing circuit 258 is in the first stateat 1015, output voltage 108 is still less than the desired outputvoltage value, secondary switch control circuit 256 then transmitsanother request signal U_(REQ) 1013 causing primary control circuit 250to set power switch 222 into the ON state and then the OFF state (e.g.in response to a turn-off condition), as indicated at 1017. The energytransferred to the secondary side of power converter 100 causes V_(OUT)108 to further increase to a value that is greater than the desiredoutput voltage value at 1018.

The above description of illustrated examples of the present invention,including what is described in the Abstract, is not intended to beexhaustive or to be limiting to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific examplevoltages, currents, times, etc., are provided for explanation purposesand that other values may also be employed in other embodiments andexamples in accordance with the teachings of the present disclosure.

What is claimed is:
 1. A power converter controller comprising: aprimary controller to be coupled to a power switch of a power converter,wherein the primary controller is coupled to receive one or more requestsignals and transition the power switch from an OFF state to an ON statein response to each of the received request signals, and wherein theprimary controller is coupled to detect a turn-off condition when thepower switch is in the ON state and transition the power switch from theON state to the OFF state in response to detection of the turn-offcondition; and a secondary controller galvanically isolated from theprimary controller, wherein the secondary controller is coupled totransmit the one or more request signals to the primary controller, andwherein the secondary controller is coupled to control an amount of timebetween the transmission of each of the request signals, the secondarycontroller including: a timing circuit that sets a minimum amount oftime between the transmission of each of the one or more requestsignals; and a secondary switch control circuit coupled to trigger thetiming circuit in response to transmitting the one or more requestsignal, wherein the timing circuit is coupled to operate in a firststate until triggered to operate in a second state by the secondaryswitch control circuit, wherein the timing circuit is in the secondstate for a holding period and transitions back to the first state atthe end of the holding period.
 2. The power converter controller ofclaim 1, wherein the primary controller is configured to be coupled to aprimary side of the power converter, wherein the primary controller iscoupled to receive the one or more request signals from a secondary sideof the power converter, and wherein the secondary controller isconfigured to be coupled to the secondary side of the power converter.3. The power converter controller of claim 1, wherein the minimum amountof time between each of the one or more request signals sets the minimumamount of time between two consecutive transitions of the power switchfrom the OFF state to the ON state.
 4. The power converter controller ofclaim 1, wherein the secondary switch control circuit is coupled tosense an output quantity of the power converter and transmit one of therequest signals to the primary controller when the sensed outputquantity is less than a desired output quantity and the timing circuitis in the first state.
 5. The power converter controller of claim 1,wherein the primary controller is coupled to maintain the power switchin the OFF state until the primary controller receives a request signal.6. The power converter controller of claim 1, wherein the turn-offcondition includes at least one of a threshold current limit and athreshold amount of time.
 7. The power converter controller of claim 1,wherein the primary controller is coupled to adjust the turn-offcondition in response to an amount of time the power switch is in the ONstate.
 8. The power converter controller of claim 6, wherein the primarycontroller is coupled to adjust the turn-off condition in response to anumber of request signals received by the primary controller.
 9. Thepower converter controller of claim 6, wherein the primary controller iscoupled to: detect an amount of current through the power switch whilethe power switch is in the ON state; determine when the amount ofcurrent through the power switch is greater than the threshold currentlimit; and transition the power switch from the ON state to the OFFstate when the amount of current through the power switch is greaterthan the threshold current limit.
 10. The power converter controller ofclaim 6, wherein the primary controller is coupled to transition thepower switch from the ON state to the OFF state when the power switchhas been in the ON state for the threshold amount of time.
 11. The powerconverter controller of claim 1, wherein the secondary controller iscoupled to transmit the one or more request signals via a communicationlink, wherein the primary controller is coupled to receive the requestsignals via the communication link, and wherein the communication linkincludes at least one of an optical communication link, a capacitivecommunication link, and a magnetic communication link.
 12. A powerconverter controller comprising: a secondary controller comprising: atiming circuit coupled to operate in a first state until triggered tooperate in a second state, wherein the timing circuit is in the secondstate for a holding period and transitions back to the first state at anend of the holding period; and a secondary switch control circuitcoupled to sense an output quantity of a power converter and transmit arequest signal when the sensed output quantity is less than a desiredoutput quantity and the timing circuit is in the first state, whereinthe secondary switch control circuit triggers the timing circuit inresponse to transmitting the request signal; and a primary controller tobe coupled to a power switch of the power converter and galvanicallyisolated from the secondary controller, wherein the primary controlleris coupled to receive the transmitted request signal and set the powerswitch into an ON state in response to the request signal, and whereinthe primary controller is coupled to detect a turn-off condition andtransition the power switch from the ON state to an OFF state inresponse to detection of the turn-off condition, the timing circuitconfigured to set a minimum amount of time between the transmission ofthe request signals.
 13. The power converter controller of claim 12,wherein the power converter is a non-isolated power converter.
 14. Thepower converter controller of claim 12, wherein the power converter isan isolated power converter, wherein the secondary controller isconfigured to be coupled to a secondary side of the power converter, andwherein the primary controller is configured to be coupled to a primaryside of the power converter.
 15. The power converter controller of claim12, wherein after transitioning the power switch to the OFF state, theprimary controller is coupled to maintain the power switch in the OFFstate until receiving another request signal.
 16. The power convertercontroller of claim 12, wherein the primary controller is coupled tomaintain the power switch in the OFF state for a threshold period oftime after transitioning the power switch to the OFF state, and whereinthe primary controller is coupled to refrain from setting the powerswitch into the ON state in response to a subsequent request signalreceived during the threshold period of time.
 17. The power convertercontroller of claim 12, wherein the holding period is selected to be avalue that is greater than an amount of time the primary controllermaintains the power switch in the ON state.
 18. The power convertercontroller of claim 12, wherein the request signal is a pulse, andwherein the primary controller is coupled to receive the pulse and setthe power switch into the ON state in response to the pulse.
 19. Thepower converter controller of claim 12, wherein the turn-off conditionincludes a threshold current limit, wherein the primary controller iscoupled to sense an amount of current through the power switch when thepower switch is in the ON state, and wherein the primary controller iscoupled to transition the power switch from the ON state to the OFFstate when the amount of current through the power switch is greaterthan the threshold current limit.
 20. The power converter controller ofclaim 12, wherein the turn-off condition includes a threshold amount oftime, and wherein the primary controller is coupled to transition thepower switch from the ON state to the OFF state when the power switchhas been in the ON state for the threshold amount of time.
 21. The powerconverter controller of claim 16, wherein the primary controller iscoupled to adjust the turn-off condition in response to at least one ofan amount of time the power switch is in the ON state and a number ofrequest signals received by the primary controller during a period oftime.
 22. A power converter comprising: an energy transfer elementcomprising a primary winding on a primary side of the power converterand a secondary winding on a secondary side of the power converter; apower switch coupled to the primary winding; a primary controllercoupled to the power switch, wherein the primary controller is coupledto receive one or more request signals from the secondary side andtransition the power switch from an OFF state to an ON state in responseto each of the received request signals, and wherein the primarycontroller is coupled to detect a turn-off condition when the powerswitch is in the ON state and transition the power switch from the ONstate to the OFF state in response to detection of the turn-offcondition; and a secondary controller coupled to the secondary side andgalvanically isolated from the primary controller, wherein the secondarycontroller is coupled to transmit the one or more request signals to theprimary controller, and wherein the secondary controller is coupled tocontrol an amount of time between the transmission of each of therequest signals, the secondary controller including: a timing circuitthat sets a minimum amount of time between the transmission of each ofthe one or more request signals; and a secondary switch control circuitcoupled to trigger the timing circuit in response to transmitting theone or more request signal, wherein the timing circuit is coupled tooperate in a first state until triggered to operate in a second state bythe secondary switch control circuit, wherein the timing circuit is inthe second state for a holding period and transitions back to the firststate at an end of the holding period.
 23. The power converter of claim22, wherein the minimum amount of time is set to a value that is greaterthan a sum of an expected ON time of the power switch and an expectedamount of time during which energy is delivered to the secondary side ofthe power converter.